Analysis of data obtained on over 50 primary murine lymphocytic neoplasms, most of which were composed of B lymphocytes, and derived lines transplanted in mice, was continued. Some of these tumors arose in the spleen and, when transplanted, homed only to the spleen. Cells of these tumors were unstable, changing their homing pattern ither during growth in the primary host or during transplantation. Because such homing changes in the primary host are almost always associated with enlargement of the gut-draining mesenteric node, it seems likely substances from the gut play a role in inducing the change. Prior to the change, cells have the characteristics of early B lymphocytes, whereas after the change demonstrate homing patterns like tumors of more mature B lymphocytes with surface immunoglobulins of the A or G isotypes.